Rotary drilling of a borehole beneath the surface of the earth is a practice typically used as part of an exploitation plan for transporting subsurface fluids, gases and minerals to the earth's surface. A “drill string” extends down the borehole and is suspended from a drilling rig. The drill string creates the borehole. At the distal end of the drill string is the “drill bit” or “bit” which removes material from the circular base of the borehole.
The action of removing this material is usually accomplished by rotating the bit about an axis that is approximately coincident with the center of the borehole. The bit is advanced towards the base of the borehole as material is removed so as to continually remove material and extend the length of the borehole. Such motion to advance the borehole is controlled at the surface by lowering the entire drill string in a controlled manner. The lowering of the drill string may be controlled by monitoring the buoyant weight of the drill string at the surface, the torque required to rotate or hold stationary the drill string, the fluid pressure of the drilling fluid or feedback from downhole telemetry.
When the axis of drill bit rotation is not coincident with the center of the borehole, the hole formed will appear to curve or change direction with respect to the previously drilled borehole. When the orientation of the drill bit rotation is intentionally misaligned with the borehole axis to effect a change in borehole direction it is commonly referred to as “directional drilling”.
A common method of drilling directionally uses a drilling fluid driven turbine or “mud motor” to rotate the drill bit. In conventional jointed tubing directional drilling a bottom hole assembly (BHA) comprises, a drilling assembly including the bit, a bent housing and the motor. The BHA is located at the downhole end of a rotary drill string. The bent housing offsets the axis of the drill bit from that of the drill string.
The use of the mud motor allows the drill bit to be rotated independently of the rest of the drill string. The entire drill string may be rotated using rotary power applied at the surface. Typical methods of applying rotational motion to the entire drill string are by the use of a “kelly drive” or “top drive” supported in a drilling rig at surface.
The mud motor drives the drill bit through the bent housing and a universal joint which allows an intentional misalignment with the axis of the borehole. This misalignment may be a set angular displacement from the mud motor axis or it may be adjustable so as to be set to a specific angle either manually at the surface or by remote telemetry when the assembly is in the borehole below the surface.
When the axis of drill bit rotation is misaligned with the axis of the mud motor and the drill string is not being rotated from the surface, the borehole formed will be curved in a manner that depends on the misalignment of these two axes.
Conventional directional drilling is accomplished with an alternating combination of two drilling operations; a period of steering or sliding; and a period of rotating. The result is a borehole with alternating straight and curved sections from the kick off point to the end of the curve. More specifically, during the sliding operation, the drill string is slowly rotated to orient the bent housing in the desired direction and drill string rotation is stopped. The mud motor is then energized so as to drill a curved path in the oriented direction. The non-rotating drill string slides along the borehole as the mud motor/drill bit drill the curved path. The sliding phase is necessary for adjusting or setting the direction of the borehole path, however this phase is somewhat inefficient due to factors including the indirect angular path and the friction or drag of the sliding drill string. Once the desired borehole inclination is established, a rotating operation commences which uses a combination of simultaneously rotating the mud motor/drill bit and the drill string (which continuously rotates the bent housing) and which favorably results in both a higher rate of penetration (ROP) and a substantially linear path.
Drilling in this manner is accomplished by supplying pressurized fluid through the center of the drill string to turn the mud motor and drill bit at the base of the hole while applying sufficient torque resistance at the surface to prevent the drill string from rotating. In the parlance of directional drilling practices, this is usually referred to as “sliding” as the only external portion of the drill string that is rotating is the drill bit. The drill bit is advanced in a manner described above such that the drill string slides without rotating along the existing borehole to advance the drill bit and maintain the action of removing material from the base of the borehole. This comprises the normal manner in which the borehole alignment can be changed with respect to vertical (referred to as the inclination and ranging from zero at vertical up to 90 degrees when horizontal) and a horizontal reference direction, usually true or magnetic north (referred to as azimuth and ranging from zero to 360 degrees with respect to the orientation of the drill string to the reference direction in the horizontal plane).
When the drill string is being rotated while the drill bit is being rotated by the mud motor, the hole is lengthened but there is little tendency to change direction. Called “rotating” in directional drilling parlance, this mode of drilling is used to advance the borehole along an axis that coincides with the axis of the mud motor which in turn roughly coincides with the line that runs through the center of borehole. Drilling in this manner serves to maintain the inclination and azimuth at constant values while the borehole is lengthened, or in more simple terms, tends to drill a straight borehole.
Turning the borehole or drilling in sliding mode requires one to prevent the drill string from rotating while maintaining or controlling the parameter used to advance the drill bit as material is removed from the base of the borehole. As the length of the borehole increases, static frictional resistance to drill string movement along the borehole also increases. This is especially true in the case of wells being drilled horizontally or at a high angle of displacement relative to vertical.
The force required to overcome the static friction resistance is typically supplied by lowering the drill string at the surface to decrease the buoyant weight of the drilling assembly carried by the surface hoisting system and thereby increasing the axial force acting along the borehole axis. The amount of force required to initiate movement of the drill string can be substantial in wells with a significant length of borehole at a high angle of displacement off vertical. Overcoming the static friction to initiate movement can result in significant drill string movement and cause problems in controlling the orientation of the bit and amount of force applied on the cutting structures of the bit. In severe cases the amount of movement of the drill string after overcoming the static friction can cause an overload on the cutting structures of the drill bit which can damage the bit, exceed the torque available to turn the bit or alter the orientation of the cutting structure so that the hole is not being curved in the desired direction.
Methodologies for minimizing the effect of static friction include U.S. Pat. No. 6,997,271 to Nichols et al. which discloses an assembly for permitting rotation slippage between a lower portion of the drill string and an upper tubular of the drill sting to thereby release torsional energy from the drill string and lessening incidences of slip-stick during drilling. In U.S. Pat. No. 5,738,178 to Williams et al, the slip-stick problem during sliding is obviated by continuously rotating the drill string while compensating at the BHA by adjusting the direction and rotational speed of the BHA to either maintain the BHA in a static position for directional drilling despite the rotating drill string, or to rotate with the drill string or independently of the drill string. This requires significant control of the BHA.
Other methodologies for mitigating the effect of static friction is to oscillate the drill string at the surface in a manner that rotates the drill string in one direction for a short distance or time followed by an equal amount of rotation in the opposite direction. The purpose of this method is to keep much of the drill string in motion, however slight, to reduce the amount of static friction to be overcome when attempting to advance the drill string along the borehole.
There are a number of patents issued for this method and they vary mainly in how the movement of the drill string is monitored and controlled. Such methodologies are described in U.S. Pat. No. 6,050,348 to Richardson et al. (Canrig Drilling Technology) U.S. Pat. No. 6,918,453 and U.S. Pat. No. 7,096,979 to Haci (Noble Drilling) and U.S. Pat. No. 7,152,696 to Jones (Comprehensive Power, Inc.). These methods of mitigating the effects of static friction, when sliding, have relied on rocking the quill of a surface swivel assembly of the drill string back and forth to induce movement in much of the drill string to reduce the amount of the static friction that must be overcome to advance the drill string as the bit removed material from the base of the borehole. Though effective, it is believed that this technique still allows the drill string to be stationary at the point of zero rotary speed, which occurs at the end of each period of rotating in one direction. One may deduce that that, every time the rotation is reversed, the static friction to induce rotation must be overcome to start rotary movement in the opposite direction. As this is controlled from surface, one might further deduce that from a stationary position, rotation in any direction will start at the surface and propagate down the borehole to the BHA so as to not affect tool-face orientation. It is believed that the amount of axial force required to overcome static friction varies constantly and that there is only a brief period during each rocking cycle when the entire desired amount of drill string is actually in motion. Axial movement is most likely to occur when the static friction is at it's lowest, which is when the maximum amount of drill string is in motion. In this manner, the drill string will be advanced in small slides at the end of each rocking sequence which is not optimal for drilling.
There is still a need for a solution for effective methods to mitigate the effect of static friction on axially advancing of the drill string when directional drilling.